The correct answer is C) in and out of a vessel that is holed in a wing tank. The reason this is the correct answer is that free-surface effect, which is the greatest influence on a vessel's stability, occurs when liquids are able to shift from side to side within a vessel's tanks. This is most pronounced when a vessel is holed in a wing tank, allowing the liquid to flow in and out of the tank. This shift in the liquid's position causes a change in the vessel's center of gravity, which significantly impacts its stability. The other options are incorrect because: A) peak tanks are located at the ends of the vessel and would not have the same side-to-side effect; B) liquids flowing fore and aft would not have the same destabilizing effect as side-to-side motion; and D) while side-to-side liquid movement affects stability, the question specifically asks about the "greatest effect," which occurs with a holed wing tank.